Aluminide coatings provide protection against oxidation and corrosion degradation to nickel and cobalt base superalloy articles used in gas turbine engines. U.S. Pat. Nos. which are indicative of the skill in the art relative to aluminide coatings include the following: 3,079,276, 3,276,903, 3,667,985, 3,801,353, 3,837,901, 3,958,047, 4,132,816, 4,142,023, 4,148,275 and 4,332,843. In general, aluminide coatings are formed by heating a powder mixture containing a source of aluminum, an activator, and an inert buffer or diluent, in the presence of the article to be coated. The article may either be embedded in the powder mixture (and the process is termed a "pack cementation" process) or the article is suspended in out-of-contact relation with the powder mixture (and the process is termed a "vapor phase" process).
The source of aluminum may be pure aluminum metal or it may be an alloy or intermetallic containing aluminum, such as Co.sub.2 Al.sub.5, as disclosed in Benden et al. U.S. Pat. No. 4,132,816; Baldi U.S. Pat. No. 3,958,047 discloses the use of Ni.sub.3 Al as the source of aluminum; and Ahuja U.S. Pat. No. 4,332,843 discloses the use of Fe.sub.2 Al.sub.5. Activators which have been used in the aluminiding process generally include halides of alkali or alkaline earth metals. See, e.g., the aforementioned patent to Benden. Aluminum oxide is the typical diluent added to the powder mixture and controls the aluminum activity of the mixture. Aluminum oxide also prevents the powder mixture from sintering together during the coating process, as discussed in Levine et al. U.S. Pat. No. 3,667,985.
Three problems which have been prevalent, especially in the gas phase aluminiding processes, are the formation of cryolite, Na.sub.3 AlF.sub.6, on the surface of the coated article; the aggregation of "zipper oxides" on the original substrate surface; and the formation of oxides within the coating itself. Cryolite has been found to accelerate the rate of base metal degradation. While cryolite formation can sometimes be limited by using special aluminiding powder mixtures, the quality of the coatings produced by such mixtures is considered to be not as good as the quality of the coatings produced by powder mixtures that result in cryolite formation. Oxides at the coating-substrate interface, and within the coating itself are undesired, since they also degrade coating properties. The former types of oxides can cause exfoliation of the coating; the latter type can act as fatigue initiation sites and sites for accelerated oxidation degradation.
Notwithstanding the advances made in the aluminiding field, researchers continue in their attempts to provide better coatings. Such coatings must have excellent resistance to oxidation and corrosion attack, and must be resistant to thermal fatigue. The present invention results from such effort.